This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Application 00-25041 filed in Republic of Korea on May 10, 2000; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method for preparing a zinc sulfide based phosphor having effective emission at low voltages.
2. Description of the Related Art
ZnS:Cu, Al phosphors, green-light-emitting sulfide based phosphors, have been extensively used in a variety of display devices. Commercially available ZnS:Cu, Al phosphors are prepared using CuSO4 as an activator, Al(NO3)3 and Al(SO4)3. 18H2O as co-activators, and ZnCl2 and NaCl as fluxes. Such sulfide based phosphors are suitable for use in display devices operated with high voltages, such as cathode ray tubes (CRTs). However, the sulfide based phosphor is inappropriate in display devices operated with low voltages.
In field emission displays (FEDs) driven at low voltages, emission of a phosphor is accelerated by a low electron energy of 1 kV or less. The penetration depth of electrons into a phosphor particle does not exceed a few nanometers, and excitation voltage and phosphor emission efficiency are low, thereby causing high current density. Since movement of charges occurs at the surface of phosphor, the surface of phosphor particle must be carefully considered, especially for the application in a low-voltage device. Thus, many approaches have been actively carried out to develop phosphors suitable for FEDs driven with low voltages.
In developing a new phosphor for a particular use, and improving the characteristics of phosphor, a variety of techniques have been adopted. For example, the characteristics of phosphor can be enhanced by appropriately choosing high-purity raw materials and fluxes, and by carefully conditioning the firing process and the post-treatment process after the synthesis of phosphor.
In addition, for utility in display devices operated with low voltages, which have the problems of low emission efficiency and high current density due to their low excitation voltage, as previously mentioned, a surface treatment technique has been applied to phosphors so as to decrease the current density.
To solve the above problems, it is an objective of the present invention to provide a method for preparing a zinc sulfide based phosphor having effective emission for use in devices operated with a driving voltage as low as 300 to 1000 eV, the phosphor having a particular composition including an activator, a co-activator and the fluxes.
The objective of the present invention is achieved by a method for preparing a zinc sulfide based phosphor having effective emission at low voltages, the zinc sulfide based phosphor comprising Cu as an activator, and Al as a co-activator, the method comprising the steps of; (a) weighing ZnS, Cul, AlF3 and at least two iodide series fluxes having a high purity, and uniformly mixing the ZnS, Cul, AlF3 and the at least two iodide series fluxes with deionized water to form a paste; (b) drying the paste and sieving to form uniform particles, and firing the particles to obtain phosphors; and (c) sieving and washing the phosphors with deionized water to remove impurities and the fluxes from the phosphors.
Preferably, the monovalent copper (Cu1+) is complexed with ammonia. The monovalent copper (Cu1+) may be mixed in an amount of 0.003 to 7.10% by weight. Preferably, in step (a), AlF3 is mixed in an amount of 0.002 to 0.15% by weight, and Nal as the iodide series metal is mixed in an amount of 0.005 to 2.0% by weight. ZnI2 as the iodide series metal may be mixed in an mount of 0.002 to 2.0% by weight. Preferably, in step (b), the particles are put into a carbon crucible and are fired in a reduction atmosphere so as to prevent the phosphors from being oxidized during the firing.